1. Field of the Invention
The invention relates generally to methods and apparatus for converting electrical signals to and from optical signals. More particularly, the invention relates to an optical fiber link card which serves as part of a communication module (not necessarily an enclosed or encapsulated device) for converting between parallel electrical signals and serial optical signals, and to a process for fabricating the module.
2. Description of the Related Art
Many types of electro-optical converters and connectors are described in issued patents and the technical literature. Commercially available devices are also presently available for performing these functions.
Examples of patents which describe electro-optical converters and connectors include U.S. Pat. No. 4,545,077, to Drapala et al, which teaches an electrical multiplex data bus operatively connected to an optical multiplex data bus by means of an electro-optical converter; and U.S. Pat. No. 4,597,631, to Flores et al, which teaches a passive electro-optical connector.
The Drapala et al invention is an example of a serial electrical to serial optical converter. Drapala et al operates as a tri-state repeater to effectively extend a three state data bus. The Flores invention is one of many connector devices that, although not dealing with electro-optical conversion per se, provides user access to transmitter/receiver assemblies via connectorized ports. The Flores device is an example of means for interconnecting both optical and electrical components in a hybrid system.
Commercially available connectorized electro-optical converters for performing serial optical to serial electrical (and vice versa) conversion, are available from Siemens and other companies. These devices, which are compatible with FDDI standards, IBM equipment, etc., are capable of achieving approximately 200 Mbits/sec. data rates.
Another example of a commercially available serial to serial converter is the AT&T ODL-200. This device is also capable of achieving approximately 200 Mbits/sec. data rates. The AT&T ODL-200 is described in an article entitled "Transmitter and Receiver Integrated Circuits for a 200 Mbits/sec. Optical Data Link", published in the proceedings of the IEEE 1986 Custom Integrated Circuits Conference.
The aforementioned commercially available devices utilize a single optical transmitter coupled through a fiber to a single receiver. Both the Siemens and AT&T devices allow full duplex communication with serial input/output. The receivers and transmitters used are hybrid ceramic substrates in dual inline packages. These packages are either hermetically sealed or plastic encapsulated.
Transceiver packages are also commercially available. For example, Mitsubishi Electric has made available transceiver packages having line bit rates of approximately 170 Mbits/sec., using a laser diode driver and optical receiver which are integrated onto a single side of a card. Similar to the aforementioned Siemens and AT&T transmitter/receiver modules, the Mitsubishi transceivers process input and output serially.
By placing the laser transmitter and receiver on the same side of a card, the Mitsubishi device requires means for electrically isolating these components. Typically metal shielding is used. Providing this isolation has heretofore limited the ability to manufacture a compact card onto which a plurality of transmitter and receiver pairs could be mounted.
The demand for improved electro-optical converters and connectors is rapidly increasing as fiber optic technologies are adapted to provide solutions to performance and packaging problems associated with present day computer interconnect applications. More particularly, I/O pin limitations caused by use of wide parallel data busses, performance limitations on the length of electrical busses, and electromagnetic interference problems, suggest that serial optical communications be used to convey data at high speed between the parallel electrical busses to which computer components are often interconnected. The high data rates required to service wide parallel data busses, the packaging flexibility of connectorized optical transmitter/receiver assemblies, and the necessity for user access to these connectorized ports, has led to the design of small feature cards to interconnect computer elements.
One such card is included in the commercially available PCO-2001 Series Parallel Lightwave Interface Module. This module performs parallel electrical signal to serial optical conversion (and vice versa) and features serial signal rates of up to approximately 100 Mbits/sec. A longwave LED is used for an optical source. Specialized transmitter and receiver ICs are incorporated onto one side of a card and provide full duplex operation.
The PCO-2001 card allows next level applications packages to interface with a high speed serial fiber optic link without adding to the design complexity of the next level packaging or performance requirements. However, the PCO-2001 card is problematic because of the electrical signal power needed for a LED source to drive data in the 200 Mbits/sec. range (twice that of the published data rate for the PCO-2001 device); the size of the PCO-2001 card (attributable in part to providing the isolation required for the transmitter and receiver components mounted on the same side of the card); and the inability to use a single, compact card to provide at least double full duplex operation.
Accordingly, it would be desirable if an optical fiber link card could be provided that supports a parallel user interface, such as a parallel data bus; performs parallel to serial conversion (and vice versa) for servicing a high speed serial optical link, where the optical transmitter on the card is capable of driving data in excess of 200 Mbits/sec. without requiring the electrical signal power needed for an LED source; and can support at least double full duplex connections in less space then is presently required to support single full duplex connections.
Furthermore, it would be desirable if the arrangement of the transmitter and receiver devices on such a card, together with the card itself, provided means to isolate transmitter and receiver electrical components without requiring excessive shielding or the amount of device separation required by the prior art.
Still further, it would be desirable if advantage could be taken of both sides of an optical data link card to increase the surface onto which components could be mounted to reduce card size. Further yet, it would be desirable to mount the optical components (and leads to these components) on the card in such a way as to facilitate easy access by a user and minimize lead capacitance and inductance to thereby further improve card performance.
To achieve the desired communication module many architecture, electrical and packaging problems need to be solved. For example, laser transmitters would be capable of achieving the desired data rates without requiring the electrical signal power required by LEDs; however, laser based systems must meet stringent safety requirements.
From a safety point of view, it would be desirable if a laser based optical fiber link card could be developed that is "fail safe", i.e., is certifiably safe at other than a total system level (where the system usually includes both hardware and software). The ability to produce a self-contained transmitter/receiver function in a certifiable package, completely independent of user system interface hardware and software, would ease restrictions on system level usage of a laser based card.
Many countries require certification of the "product" with respect to laser light emissions. Prior art laser based optical link subassemblies have a dependency on the "box" they are in to maintain compliance. If all the laser safety circuitry were on board the optical fiber link card, then the card would become the "product" that needs to be certified; not all the different models of boxes that it is used in. This would simplify the safety certification process for the user. More particularly, it would be desirable if an optical fiber link card could be devised that maintains known worldwide standards for class 1 operation under a single component failure.
In addition to all of the above, it would be desirable if an optical fiber link card communication module containing the features described hereinbefore also (1) provides a byte sync signal to the user, since many optical link subassemblies deliver fragmented parallel data; (2) provides a fault line to the user to aid in determining which end of an optical link suffers a failure; (3) provides an electrical wrap capability for diagnostic purposes; (4) requires only a single +5 volt supply which would make the card compatible with single voltage logic families; (5) maintains good thermal isolation between the electronics and the laser; (6) provides a package adaptable to multiple next level packaging; (7) uses standard surface mount assembly techniques instead of the expensive ceramic hybrid hermetic packaged subassemblies (particularly for the optical drivers and receivers) used by the prior art to achieve high data rates; and (8) is compact, i.e., small in size and height compared to known systems.